I don't wish for anything I say to be read in a harsh or trollish tone. I really want to engage in a productive dialog.

I'll start from the top down with your post and explain how I feel and think:

Well I don't think Bigfoot exists either. But falsifying that claim would require very careful investigation of the whole Maine landscape(or wherever Bigfoot supposedly lives). In contrast, you could investigate your mind by taking high doses of tryptamines(http://countyourculture.com/2012/03/10/n-alkylated-tryptamin...) in a safe space and seeing if your experiences jived with anything Grof claims, or not. By the way the above url is very focused on quantitative information and the chemistry of some psychoactive compounds - no woo there don't worry. So Bigfoot is not a good analogy because investigating Grof's claims is an obtainable goal, not an impossibility.

I don't intend this to come off as snarky or rude, but I've been reading rationality literature myself for many years. Dawkins, Dennett, Russel, some Hume, LessWrong, etc. I've actually used Russell's teapot myself as a conceptual metaphor while arguing with naive religious people. I'm also a math major so I'm not exactly immune to logical thought. So please hear me out.

It seems like you are being triggered by the word astrology, this is very understandable and I normally associate astrology with bullshit myself. Let me shift my attention to some of the other things you wrote about experiential evidence.

Here is my take on experiential evidence. All evidence is experiential in the sense that we must use our sensory organs to use any machine or observe the world 'directly' but of course we really experience only what our brains construe from sensory bombardment of light, sound, etc. All this is quite orthodox so far as I hope you'll agree.

So, with various methods, high doses of tryptamines being the easiest and most reliable, humans can experience a variety of states that feel just as real, indeed often "realer than real" as everyday sensory reality. So by what basis except gut level feeling do we disbelieve in the reality of dreams and believe in the absolute existence of waking sensation? It's really only that dreams feel vague and are hard to hold on to mentally and the real world is bold and seemingly objective because other people agree they experience it in similar ways. So much for the reality of the world and the unreality of dreams.

But there is a third category of experience available to humans and that is a visionary state. In such non ordinary states of consciousness people often have experiences that correlate with the experiences of others. So in this way it is similar to the situation with waking reality - independent verification of a 'territory' except that one is material and one is experiential/internal.

Secondly, as I've said before, often these experiences feel realer than real. Thus there is no reason to dismiss them out of hand, but rather treat reports and self-investigation as evidence of the same class as evidence from the material world. That said, we must be careful to avoid mistakes and confusion and make false theories based on evidence regardless of its kind.

Grof has taken large doses of LSD with sensory isolation hundreds of times and he has observed, for the full duration of the experience, over 5000 therapeutic sessions where LSD was used. He also had colleagues that did similar work and had access to their notes. In addition, many of the things Grof claims are compatible to a large degree with other researches who have used visionary plants and chemicals.

I myself have had several experiences that led me to find some of Grof's ideas credible. Specifically Grof thinks humans can undergo a state of 'cosmic consciousness', or identification with the Godhead, unitive experience, there are many names for it. I know for a fact this can happen because it happened to me.

Back to astrology. I admit I'd call bullshit in almost every instance I see the term 'astrology' but I give Grof a bit of benefit of the doubt. For one thing I believe he probably does not mean that the actual planets materially influence human affairs. But rather that some sort of periodic system that is analogous to the movements of the planets influences the internal world of our minds. I have no idea if this is true or not.

I think it's quite likely that Grof, as a modern pioneer of this type of research, has gotten some things wrong. I also think that he's on the right track to a much greater degree than conventional psychotherapists and psychologists with regards to the inner workings of the mind.

Why do I think this? Because of my lived experience both in normal waking life and in NOSC. Also conversations with friends.
Why does Grof think this? Because of his own extensive experience with NOSC and his work with thousands of therapy clients.

If that's not empirical then I don't know what is.

However, I would have completely rejected my own argument and taken your position years ago before I had sufficient experience with NOSC. So I can sympathize if even this post does not convince you.

Edit:
To use one of the standard atheist stories for my own benefit, consider the whole telescope and Galileo and the Church incident. Supposedly some thought that they mustn't look at the sky through the telescope and thus could dismiss any claims made by those who chose to look. Clearly in that case those who used the telescope had a privileged position, more accurate, and indeed incomprehensible by non-lookers. So I submit to you: NOSC are telescopes/microscopes for our interior world and if you only use your rational/cognitive mind you're essentially missing half of the picture of what it means to be human.